Clinical manifestations of cardiovascular disease, such as heart attack, stroke, and congestive heart failure, are the number one cause of death in the developed world. Despite considerable interest in specific cardiovascular risk factors such as hypercholesterolemia, diabetes, and hypertension, data from the Framingham Heart Study indicate clearly the main determinant of incident cardiovascular disease is aging. Aged blood vessels are characterized by reduced control of antioxidant enzymes, an increased flux of reactive oxygen species, and vascular dysfunction. This flux of reactive oxygen species has proven to be a major determinant of vascular phenotype as a function of age, consistent with the "Free Radical Theory" of aging. This theory, originally proposed by Harman/1, pointed to the mitochondrion as the principal source of free radicals. However, there is now compelling evidence for multiple enzymatic sources of reactive oxygen species throughout blood vessels and particularly in the endothelium. Principle among these are the NADPH oxidase family of enzymes that are related to the classical neutrophil respiratory burst oxidase. We and others have found that endothelial cells contain two specific isoforms of the NADPH oxidase catalytic subunit (Nox);Nox2 and Nox4. The former is identical to the respiratory burst oxidase (GP91/phox, now Nox2), whereas the latter is novel. Considerable data indicate that excess vascular reactive oxygen species that are characteristic of vascular disease and aging are due, in part, from Nox enzymatic activity. Most investigation on Nox-derived reactive oxygen species and vascular disease has focused on Nox2. However, the role of Nox4 and how it differs with Nox2 with regard to vascular phenotype is not known. This is all the more problematic considering that Nox4 is the major Nox isoform in the vasculature. This proposal is designed to bridge this gap in knowledge and is based upon the hypothesis that reactive oxygen species produced by specific Nox isoforms is an important determinant of the maladaptive vascular phenotype that is characteristic of vascular disease and aging. The objective of this application, therefore, is to determine the role of specific Nox isoforms in controlling endothelial cell phenotype and elucidate the mechanisms involved. In order to achieve this objective, we will first characterize the NADPH oxidase catalytic subunit isoforms in endothelial cells with expect to their expression, subcellular localization, and subunit requirements. We will then use gain-of-function and loss-of-function strategies to determine the role of Nox isoforms in controlling endothelial cell phenotypes involving ROS such as NO bioactivity, adhesion molecule expression, proliferation, migration, and senescence. We will then go on to manipulate Nox4 expression in mice using stable siRNA expression or cell-specific Nox overexpression and examine the implications for vascular function and the response to arterial injury. Successful prosecution of these studies will provide mechanistic information on redox-mediated control of vascular cell phenotype and afford us the necessary insight to design new therapeutic strategies that focus on Nox-derived reactive oxygen species.